U.S. patent number 5,531,221 [Application Number 08/304,046] was granted by the patent office on 1996-07-02 for double and single acting piston ventilators.
This patent grant is currently assigned to Puritan Bennett Corporation. Invention is credited to Edwin B. Merrick, John J. O'Mahony, John S. Power.
United States Patent |
5,531,221 |
Power , et al. |
July 2, 1996 |
Double and single acting piston ventilators
Abstract
The piston ventilators include a double acting reciprocating
piston, in which the piston motion delivering breathing gas to a
patient receiving assistance from the ventilator simultaneously
draws in breathing gas from a source of breathing gas, and a single
acting piston ventilator, in which only the delivery end of the
cylinder is provided with inlet and exhaust valves. In the double
acting piston ventilator, breathing gas is supplied to both ends of
the piston cylinder through two intake valves, and exhausting
breathing gas through two exhaust valves. Both intake valves of the
cylinder are ducted to the breathing gas supply, and the two
exhaust valves of the cylinder are ducted to a gas delivery limb of
the ventilator connected to the patient's airway.
Inventors: |
Power; John S. (Galway,
IE), O'Mahony; John J. (Galway, IE),
Merrick; Edwin B. (Stow, MA) |
Assignee: |
Puritan Bennett Corporation
(Carlsbad, CA)
|
Family
ID: |
23174816 |
Appl.
No.: |
08/304,046 |
Filed: |
September 12, 1994 |
Current U.S.
Class: |
128/205.18;
128/204.18; 128/204.21; 417/259 |
Current CPC
Class: |
A61M
16/0072 (20130101); A61M 16/106 (20140204) |
Current International
Class: |
A61M
16/00 (20060101); A61M 016/00 () |
Field of
Search: |
;128/204.18,204.21,205.18,203.12,204.25,204.28,205.13,205.14,205.15,205.16
;417/259,262,242,267,315 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
1319175 |
|
Jun 1993 |
|
CA |
|
1271902 |
|
Jul 1968 |
|
DE |
|
2-102383 |
|
Apr 1990 |
|
JP |
|
157638 |
|
Oct 1932 |
|
CH |
|
1555957 |
|
Nov 1979 |
|
GB |
|
Other References
Younes, et al., "Proportional Assist Ventilation"; Am Rev Respir
Dis 1992; 145: 121-129. .
Younes, et al., "An apparatus for altering the mechanical load of
the respiratory system", J. Appl. Physiol. .62(6): 2491-2499,
1987..
|
Primary Examiner: Burr; Edgar S.
Assistant Examiner: Raciti; Eric P.
Attorney, Agent or Firm: Fulwider Patton Lee &
Utecht
Claims
What is claimed is:
1. A single acting piston ventilator system for providing breathing
gas to a patient airway, comprising:
a source of said breathing gas for providing a supply flow of said
breathing gas;
a fixed volume piston cylinder having a first end and a second end,
a first inlet connecting said first end of said piston cylinder in
fluid communication with said source of breathing gas for receiving
said supply flow of breathing gas, a second inlet connecting said
second end of said piston cylinder in fluid communication with said
source of breathing gas for receiving said supply flow of breathing
gas, said first and second inlets being connected in fluid
communication, and an outlet connected to said first end of said
piston cylinder for delivering mixed gas to the patient airway;
a single inlet valve, said inlet valve located in said first inlet
for allowing flow of said mixed gas into said piston cylinder;
outlet valve means in said outlet for allowing a flow of said mixed
gas to the patient airway;
a reciprocating piston disposed within said piston cylinder and
moveable between said first and second ends of said piston
cylinder; and
means for moving said piston between said first and second
ends.
2. The piston ventilator system of claim 1, further including
control means for controlling said means for moving said piston for
controlling pressure and flow of breathing gas supplied by the
piston cylinder to the patient airway.
3. The piston ventilator system of claim 2, wherein said single
inlet valve comprises an actively controlled valve controlled by
said control means.
4. The piston ventilator system of claim 1, wherein said single
inlet valve comprises a check valve.
5. The piston ventilator system of claim 1, wherein said outlet
valve means comprises a check valve.
6. The piston ventilator system of claim 2, wherein said outlet
valve means comprises an actively controlled valve connected to and
controlled by said control means.
7. A single acting piston ventilator system for providing breathing
gas to a patient airway, consisting essentially of:
a source of said breathing gas for providing a supply flow of said
breathing gas;
a fixed volume piston cylinder having a first end and a second end,
a first inlet connecting said first end of said piston cylinder in
fluid communication with said source of breathing gas for receiving
said supply flow of breathing gas, a second inlet connecting said
second end of said piston cylinder in fluid communication with said
source of breathing gas for receiving said supply flow of breathing
gas, said first and second inlets being connected in fluid
communication, and an outlet connected to said first end of said
piston cylinder for delivering mixed gas to the patient airway;
inlet valve means in said first inlet for allowing flow of said
mixed gas into said piston cylinder;
outlet valve means in said outlet for allowing a flow of said mixed
gas to the patient airway;
a reciprocating piston disposed within said piston cylinder and
moveable between said first and second ends of said piston
cylinder; and
means for moving said piston between said first and second
ends.
8. The piston ventilator system of claim 7, wherein said inlet
valve means comprises a check valve.
9. The piston ventilator system of claim 7, wherein said outlet
valve means comprises a check valve.
10. A single acting piston ventilator system for providing
breathing gas to a patient airway, consisting essentially of:
a source of said breathing gas for providing a supply flow of said
breathing gas;
a fixed volume piston cylinder having a first end and a second end,
a first inlet connecting said first end of said piston cylinder in
fluid communication with said source of breathing gas for receiving
said supply flow of breathing gas, a second inlet connecting said
second end of said piston cylinder in fluid communication with said
source of breathing gas for receiving said supply flow of breathing
gas, said first and second inlets being connected in fluid
communication, and an outlet connected to said first end of said
piston cylinder for delivering mixed gas to the patient airway;
inlet valve means in said first inlet for allowing flow of said
mixed gas into said piston cylinder;
outlet valve means in said outlet for allowing a flow of said mixed
gas to the patient airway;
a reciprocating piston disposed within said piston cylinder and
moveable between said first and second ends of said piston
cylinder;
means for moving said piston between said first and second ends;
and
control means for controlling said means for moving said piston for
controlling pressure and flow of breathing gas supplied by the
piston cylinder to the patient airway.
11. The piston ventilator system of claim 10, wherein said inlet
valve means comprises an actively controlled valve controlled by
said control means.
12. The piston ventilator system of claim 10, wherein said outlet
valve means comprises an actively controlled valve connected to and
controlled by said control means.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to ventilators for delivering
breathing gas to the lungs of a patient, and more particularly
concerns a double acting reciprocating piston ventilator having a
piston that is supplied with breathing gas on each stroke of the
piston, automatically charging itself whenever breath support is
delivered, and requiring no retraction time between breaths. A
single acting piston is also disclosed that automatically draws
breathing gas from a source only during delivery of breathing
gas.
2. Description of Related Art
Medical ventilators are generally designed to ventilate a patient's
lungs with breathing gas to assist a patient in breathing when the
patient is somehow unable to adequately breath without assistance.
Pressure assistance often can be instituted when the patient has
already begun an inspiratory effort, typically by bellows or fixed
volume piston type ventilators.
Conventional piston lung ventilators commonly use a single action
piston, in which the gas to be delivered is drawn into a cylinder
by the retraction of the piston and is subsequently delivered to
the patient by advancing the piston. The supply flow rate during
the retraction time of a .single action piston can be much higher
than the patient's peak flow demand. It would be desirable to
provide a piston ventilator requiring a supply flow rate only as
great as the rate required by the patient.
Such piston lung ventilators, while typically having piston seals,
may allow small volumes of breathing gas with a high oxygen
concentration to leak past the seals, to escape into the interior
of the ventilator. A piston ventilator using an unsealed piston is
also known that results in a higher volume of leakage. For example,
in the known unsealed piston ventilator, the piston cylinder has a
volume of about 2.7 liters, allowing for as much as 0.7 liters of
compensation for leakage past the piston and volume lost due to the
compressibility of the breathing gas in the cylinder and airway,
while still maintaining the ability to deliver a breath of up to
2.0 liters inspired volume in one stroke of the piston. It is
unsafe to allow high concentrations of oxygen to accumulate in the
interior of an electrical product, due to the risk of fire, and it
is therefore necessary with such single action ventilator systems
to scavenge any gas mixture that has leaked past the piston outside
the ventilator enclosure. The breathing gas mixture that remains in
the piston to be delivered to the patient can also be diluted by
the leakage, or "blow by" of room air that leaks past the piston
during the time the piston is retracting, drawing breathing gas
into the piston cylinder. It would be desirable to provide a piston
ventilator in which any leakage of air past the piston to dilute
the breathing gas mixture is avoided, and in which any leakage of
the breathing gas mixture past the piston is retained within the
piston cylinder, so that it does not need to be scavenged, and can
be delivered to the patient.
In addition, since such single acting pistons have a fixed volume,
in order for the ventilator to deliver a breath larger than the
volume of the piston cylinder, the single acting piston must first
advance, reverse direction, and then retract before advancing
again, causing a significant interruption of flow in the time that
is necessary for the single acting piston to cycle back to a "home"
retracted position to deliver breath support to a patient. It would
thus be desirable to provide a piston ventilator having a piston
that automatically charges itself whenever breath support is
delivered, and that can deliver a breath larger than the piston
cylinder volume when required, without an appreciable interruption
of flow, simply by reversing the direction of travel of the piston,
by the use of multiple strokes of the piston, requiring no
retraction time and a reversal time of only milliseconds between
strokes of the piston to deliver breath support. A first embodiment
of the present invention meets these needs, while a second
embodiment of the invention provides gas scavenging and reduction
of peak flow from the breathing gas supply.
SUMMARY OF THE INVENTION
Briefly, and in general terms, a first embodiment of the present
invention provides for a ventilator system with a double acting
reciprocating piston, in which the piston motion delivering
breathing gas to a patient receiving assistance from the ventilator
simultaneously draws in breathing gas from a source of breathing
gas. This is accomplished by supplying breathing gas to both ends
of the piston cylinder through two intake valves, and exhausting
breathing gas through two exhaust valves. Both intake valves of the
cylinder are ducted to the breathing gas supply, and the two
exhaust valves of the cylinder are ducted to a gas delivery limb of
the ventilator connected to the patient's airway. Breathing gas
only needs to be supplied to the piston cylinder from the source of
breathing gas at the rate required by the patient, and the problems
of scavenging oxygenated breathing gas that has leaked past the
piston and dilution of breathing gas by room air are eliminated. In
the first preferred embodiment, volumes of breathing gas larger
than the volume of the piston cylinder can also be delivered to the
patient without interruption, allowing a ventilator incorporating
the double acting piston to be constructed very compactly, such as
would be suitable for use in emergency vehicles.
The first embodiment accordingly provides for a double acting
piston ventilator system for providing breathing gas to a patient
airway. The double acting piston ventilator system comprises a
source of the breathing gas for providing a supply flow of the
breathing gas, and a fixed volume piston cylinder having a
reciprocating piston. Each end of the piston cylinder has an inlet
valve connected to the source of breathing gas for receiving the
flow of breathing gas, and each end has an outlet for delivering
breathing gas to the airway.
The first embodiment of the invention also provides a method for
supplying breathing gas to a patient airway from the ventilator
system by providing a supply flow of the breathing gas to both ends
of the piston cylinder, and actuating the piston to deliver a flow
of the breathing gas from an end of the piston cylinder to the
patient airway while simultaneously drawing the breathing gas into
the other end of the piston cylinder. A preferred aspect of the
method involves alternatingly actuating the piston to deliver a
flow of the breathing gas from the first end of the piston cylinder
while simultaneously drawing breathing gas into the second end of
the piston cylinder, and actuating the piston to deliver a flow of
the breathing gas from the second end of the piston cylinder while
simultaneously drawing breathing gas into the first end of the
piston cylinder.
In a second embodiment of the invention, only the delivery end of
the cylinder is provided with inlet and exhaust valves. The inlet
valve of the delivery end of the cylinder is connected to the other
end of the cylinder, the reservoir end, having no associated
valves. The exhaust valve of the delivery end of the cylinder is
connected to the airway. The second embodiment of the invention
provides for the containment and utilization of any breathing gas
that leaks past the piston within the cylinder and requires a
volume flow rate from the supply of breathing gas which is nearly
equal to the volume flow rate which is delivered to the patient
being ventilated.
These and other aspects and advantages of the invention will become
apparent from the following detailed description, and the
accompanying drawings, which illustrate by way of example the
features of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of a prior art piston ventilator;
FIG. 2 is a diagram of a first embodiment of the piston ventilator
of the invention;
FIG. 3 is a diagram of a second embodiment of the piston ventilator
of the invention;
FIG. 4 shows an alternate embodiment of the piston ventilator of
the invention similar to that of FIG. 2;
FIG. 5 shows an alternate embodiment of the piston ventilator of
the invention similar to that of FIG. 4; and
FIG. 6 shows another alternate embodiment similar to that of FIG. 5
with a transfer valve disposed in the piston.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In single action piston ventilators, the supply flow rate during
piston retraction can be much higher than the patient's peak flow
demand. Single action piston ventilators may also allow oxygenated
breathing gas to escape from the cylinder if it is not adequately
sealed, necessitating additional equipment and safety precautions
for scavenging the escaped breathing gas, and may also allow
breathing gas being delivered to the patient to be diluted by room
air. In addition, in delivering a volume of breathing gas larger
than the volume of the piston cylinder, single acting pistons must
first advance, reverse direction, and then retract before advancing
again, causing a significant interruption of flow of breathing gas
to the patient.
As is illustrated in the FIG. 1, a typical prior art single action
piston ventilator 10 includes a fixed volume piston cylinder 12
having a first gas delivery portion 14 with an inlet 16 for
receiving mixed breathing gas and an outlet 18 for delivering the
mixed breathing gas to the patient airway 20 during an inspiratory
portion of a breath cycle. The inlet includes a check valve 22
allowing one way flow of the mixed gas into the piston cylinder,
and the outlet 18 similarly has a check valve 24 allowing one way
flow of the mixed gas to the patient airway. A reciprocating piston
26, mounted to a piston rod 28 for moving the piston, is disposed
within the piston cylinder, and is movable within the piston
cylinder between an extended position 30 in the first gas delivery
portion 14 of the piston cylinder and a retracted position 32 in a
second portion 34 of the piston cylinder on the opposite side of
the piston from first gas delivery portion 14 of the piston
cylinder. The second portion 34 of the piston cylinder includes a
vent 36, typically with a filter 38, open to the atmosphere,
through which leaked breathing gas can escape and room air can
enter the piston cylinder.
With reference to FIG. 2, in a first embodiment the invention
comprises a double acting piston ventilator 40 for providing
breathing gas to a patient airway 42. A supply flow of breathing
gas is provided to the double acting piston ventilator from a
source of the breathing gas 44 through supply flow ducting 46
connected for fluid communication with a fixed volume piston
cylinder 48. The piston cylinder has a first inlet 50 connected to
the supply flow ducting at a delivery end 52 of the piston
cylinder, and a second inlet 54 connected to the supply flow
ducting at the other end 56 of the piston cylinder, for receiving
the supply flow of breathing gas. Two outlets are provided in the
piston cylinder for delivering mixed gas to the patient airway. As
is illustrated in FIG. 2, a first outlet 58 is connected to the
delivery end of the piston cylinder, and a second outlet 60 is
connected to the second end of the piston cylinder, with each of
the outlets being connected through outlet ducting 62 to the
patient airway.
Inlet valve means are also preferably provided in each cylinder
inlet to control the flow of the breathing gas into the piston
cylinder, and in the first preferred embodiment illustrated in FIG.
2, a first check valve 64 is provided in the first inlet, and a
second check valve 66 is provided in the second inlet, to allow
flow of the supply of breathing gas into the piston cylinder
through the inlets and to prevent backflow out of the piston
cylinder through the inlets. Outlet valve means are also preferably
provided in the outlet path to the patient airway to control the
flow of the breathing gas from the piston cylinder to the patient
airway. A first check valve 68 is provided in the first outlet, and
a second check valve 70 is provided in the second outlet, to allow
flow of breathing gas to the patient airway and to prevent backflow
from the patient airway to the piston cylinder through the
outlets.
A reciprocating piston 72 is disposed within the piston cylinder,
and is movable between a first position 74 and a second position 76
near the ends of the piston cylinder for delivering breathing gas
to the patient airway through the outlets. The reciprocating piston
is typically mounted to a piston rod 78, and linear bearings 80 are
currently preferably provided for the piston rod to ride on at both
ends of the piston cylinder. The piston rod is preferably connected
to and driven by means for moving the piston between the first and
second positions, such as a rack and pinion motor 82 controlled by
a control unit 84 such as a microcontroller or microprocessor for
controlling the pressure and/or flow of breathing gas supplied by
the piston chamber to the patient airway, so as to end the stroke
of the piston at a precise volume, or to control the pressure
profile of breathing gas delivery, for example. In order to
minimize friction, conventional seals are not used. The system is
designed to provide a small gap between the piston rod and the
bearing housing to limit the leakage to acceptable levels.
In the first preferred embodiment, the piston cylinder of the
double acting piston ventilator can typically be reduced in volume
to about a quarter of the volume of conventional piston
ventilators, allowing for a much more compact ventilator design
that can be made more easily portable, such as for use in emergency
vehicles, as it is not necessary for the piston to cycle back to a
"home" retracted position for delivery of breath support to the
patient. A breath larger than the cylinder volume can be delivered
without an interruption of flow, simply by reversing the direction
of travel of the piston, by the use of multiple strokes of the
piston. Since the piston requires no retraction time between
breaths, and requires minimal reversal time on the order of 20-30
milliseconds, breathing gas can be delivered to the patient on
demand, and without significant interruption.
In addition, when a volume of gas is expelled from one end of the
cylinder, an equal volume of breathing gas is drawn into the other
end of the piston cylinder, automatically charging itself whenever
breath support is delivered, so that the gas supply flow only needs
to be made available at the rate required by the patient. Any "blow
by" breathing gas that leaks past the piston is retained within the
piston cylinder and can be delivered to the patient. Dilution of
breathing gas by room air leaking past the piston is prevented,
allowing for delivery of up to 100% oxygen to the patient, since
the correct mixture of breathing gas is provided on both sides of
the piston, and additional dilution by room air is not
possible.
In a second preferred embodiment illustrated in FIG. 3, the design
of the piston ventilator can also be economized, by employing check
valves at only one end of the piston cylinder, rather than
providing two check valves to both ends of the cylinder. In this
embodiment, a single acting piston ventilator 90 provides breathing
gas to a patient airway 92, receiving a supply flow of breathing
gas from a source 94 through supply flow ducting 96 connected for
fluid communication with a fixed volume piston cylinder 98. The
piston cylinder has a first inlet 100 connected to the supply flow
ducting at a delivery end 102, and a second inlet 104 connected to
the supply flow ducting at a reservoir end 106 of the piston
cylinder. In this embodiment, however, only one outlet 108 is
provided in the piston cylinder for delivering mixed gas to the
patient airway, and is provided with a check valve 110 to allow
flow of breathing gas to the patient airway and to prevent backflow
from the patient airway to the piston cylinder through the
outlet.
As is shown in FIG. 3, a check valve 112 is also provided in only
the piston cylinder inlet located at the same side as the valved
outlet, to control the flow of the breathing gas into the piston
cylinder, so as to allow flow of the supply of breathing gas into
the piston cylinder through the inlets and to prevent backflow out
of the piston cylinder through the inlets while breathing gas is
being delivered through the piston outlet to the patient airway. A
reciprocating piston 114 is disposed within the piston cylinder,
and is movable between a first position 116 and a second position
118 near the ends of the piston cylinder for delivering breathing
gas to the patient airway through the outlets. As in the first
embodiment, the reciprocating piston is typically mounted to a
piston rod 120, and linear bearings 122 are also provided in the
piston cylinder for the piston rod, at both ends of the piston
cylinder. The piston rod is preferably driven by means for moving
the piston between the first and second positions, such as a rack
and pinion motor 124 controlled by a control unit 126 such as a
microcontroller for controlling the pressure and/or flow of
breathing gas supplied by the piston chamber to the patient
airway.
In the second preferred embodiment, while the piston delivers
breathing gas through the outlet check valve to the patient airway,
the breathing gas mixture is simultaneously drawn into the piston
cylinder through the inlet at the other end of the piston cylinder.
During piston retraction, the supplied gas mixture is readily
transferred through the gas supply flow duct from the end of the
piston cylinder without a check valve to the delivery end of the
piston cylinder. While breathing gas is not delivered to the
patient's airway with both strokes of the piston, breathing gas is
required to be delivered to the cylinder only during breathing gas
delivery to the patient and only at a flow rate near the flow rate
delivered to the patient. In addition, since no vent to the
atmosphere is required by this design, gas leaking past the piston
does not need to be scavenged, and can be delivered to the patient.
Room air is also not permitted to leak past the piston to dilute
the gas mixture, so that up to 100% oxygen can be delivered when
needed, without dilution.
It should be recognized that the passive check valves in the
ventilator system can also be replaced by actively controlled
valves. Thus, the check valves 64, 66, 68 and 70 of FIG. 2 can be
replaced by actively controlled valves 64', 66', 68' and 70', such
as solenoid valves or pneumatically actuated valves connected to
and controlled by the control unit 84, as is illustrated in FIG. 4.
Similarly, as is shown in FIG. 5, actively controlled valves 110'
and 112', such as solenoid valves or pneumatically actuated valves
connected to and controlled by the control unit 126, can be
substituted for the check valves 110 and 112 of FIG. 3.
In operation, as the piston travels in a direction so as to
compress the breathing gas contained in the delivery end of the
cylinder, breathing gas passes through the exhaust valve into the
airway. Simultaneously, breathing gas is drawn from the supply of
breathing gas into the reservoir side of the cylinder, which is
caused to expand in volume, by an amount nearly equal to the volume
expelled through the exhaust valve by the piston. Thus the flow
rate into the reservoir end of the cylinder is nearly equal to the
flow rate of the breathing gas expelled through the exhaust valve
from the delivery end of the cylinder. At the end of the patient's
inspiration, the piston is caused to move in a direction such as to
compress the breathing gas contained in the reservoir end of the
cylinder. This constitutes the second stroke of the piston. During
the second stroke of the piston, breathing gas is transferred from
the reservoir end of the cylinder, through the inlet valve of the
delivery end of the cylinder, into the delivery end of the
cylinder. During the second stroke of the piston no appreciable
flow from the source of breathing gas occurs, and there may in fact
be a small reverse flow equal to the expansion of the gas remaining
in the delivery end of the cylinder as it returns to a pressure
equal to the breathing gas supply pressure.
In both methods of the invention for supplying breathing gas to a
patient airway from a ventilator system as described above, a
supply flow of the breathing gas is provided to at least one of the
first and second ends of the piston cylinder, and the piston is
actuated to deliver a flow of the breathing gas from at least one
of the first and second ends of the piston cylinder to the patient
airway, while simultaneously drawing the breathing gas into the
other of the first and second ends of the piston cylinder. In the
embodiment of FIG. 2, the piston can be alternatingly actuated in
one direction to deliver a flow of the breathing gas from the first
end of the piston cylinder while simultaneously drawing breathing
gas into the second end of the piston cylinder, and then actuated
in the opposite direction to deliver a flow of the breathing gas
from the second end of the piston cylinder while simultaneously
drawing the breathing gas into the first end of the piston
cylinder.
It has thus been demonstrated that the apparatus and methods of the
invention provide for a double acting piston ventilator and a
single acting piston ventilator in which breathing gas is supplied
to the cylinder at a flow rate limited to near the patient's
required flow of breathing gas. The problem of containment and
utilization of breathing gas leaked past the piston is eliminated.
The construction of the double acting piston ventilator allows the
piston motion to simultaneously draw breathing gas from the source
of breathing gas, and to deliver breathing gas to the patient
airway. In one preferred embodiment, volumes of breathing gas
larger than the volume of the piston cylinder can be delivered to
the patient without interruption.
It should be apparent that various alternate configurations of the
embodiments of the piston ventilator of the invention are possible.
For example, as is illustrated in FIG. 6, in a variation of the
second embodiment, one or more one-way transfer valves 130 could
also be mounted in the piston itself, to provide for the gradual
filling of one side of the cylinder while the other side is
delivering flow, followed by rapid retraction not limited by the
flow rate of the breathing gas supply. In addition, the piston can
be sealed or unsealed, and a single bearing can be provided at one
end of the piston rod. The control unit can also comprise an analog
circuit instead of a microprocessor.
It will thus be apparent from the foregoing that while particular
forms of the invention have been illustrated and described, various
modifications can be made without departing from the spirit and
scope of the invention. Accordingly, it is not intended that the
invention be limited, except as by the appended claims.
* * * * *